1. Field of the Invention
The present invention relates to a process for producing an electrical cable, particularly for high voltage direct current transmission or distribution.
More particularly, the present invention relates to a process for preparing an electrical cable, particularly for high voltage direct current transmission or distribution, which is suitable for either terrestrial or submarine installation, comprising the stage of producing at least one insulating coating for said cable by hot cross-linking of a polymeric composition comprising a polyethylene, a radical initiator and a small amount of an unsaturated carboxylic acid.
The present invention moreover relates to a cable for high voltage direct current transmission or distribution in which the insulating coating consists of the abovementioned polymeric composition.
For the purposes of the present description and the claims, the term “high tension” means a tension of greater than 35 kV.
2. Description of the Related Art
The cables generally used for high voltage direct current transmission, either along terrestrial lines or, particularly, along submarine lines, are cables commonly known in the art, such as mass-impregnated cables in which the conductor, coated with a first semiconducting layer, is electrically insulated by being wound with an insulating material, generally paper or paper/polypropylene/paper multilayer laminates, which is then totally impregnated with a mixture with high electrical resistivity and high viscosity, generally a hydrocarbon oil containing a viscosity-increasing agent. The cable then comprises a further semiconducting layer and a metal screen, generally made of lead, which is itself surrounded by at least one metal armouring structure and by one or more plastic protective sheaths.
Although mass-impregnated cables are characterized by high reliability in operation even at very high voltages (greater than 150 kV), they have a number of drawbacks mainly associated with migration of the insulating fluid inside the cable. Particularly, during use, the cable is subjected, owing to variations in the intensity of the current transmitted, to thermal cycles which cause migrations of the fluid in the radial direction. As a matter of fact, when the current carried increases and the cable heats up, the viscosity of the insulating fluid decreases and the fluid is subjected to a thermal expansion greater than all the other components of which the cable is made. This leads to migration of the fluid from the insulating layer towards the exterior and, consequently, to an increase in the pressure exerted on the metal screen, which is deformed in the radial direction. When the current carried decreases and the cable cools down, the impregnating fluid contracts, whereas the metal screen, which is made of a plastic material (usually lead), remains permanently deformed. This therefore results in a decrease in the internal pressure of the cable, leading to the formation of microcavities in the insulating layer with a consequent risk of electric discharges and, hence, of perforation of the insulation. The risk of perforation increases as the thickness of the insulating layer increases and, hence, as the maximum voltage for which the cable was intended increases.
Another solution for high voltage direct current transmission consists of cables with fluid oil, in which the insulation is provided by a pressurized oil of low viscosity and high electrical resistivity (under a hydrostatic head). Although this solution is highly effective in terms of avoiding the formation of microcavities in the cable insulation, it has a number of drawbacks mainly associated with the complexity of construction and, particularly, results in a limitation of the maximum permissible length of the cable. This limitation of the maximum length is a major drawback, especially as regards submarine use, in which the lengths required are usually very great.
For many years, research has been directed towards the possibility of using cross-linked polyolefins, and particularly cross-linked polyethylene (XLPE), to produce insulating materials for cables for direct current transmission. Insulating materials of this type are already widely used in the case of cables for alternating current transmission. The use of said insulating materials also in the case of cables for direct current transmission would allow said cables to be used at higher temperatures, for example at 90° C. instead of 50° C., compared with the mass-impregnated cables described above (higher working temperatures, making it possible to increase the amount of current transported) and would eliminate limitations in the maximum permissible length of the cable, in contrast with the cables containing fluid oil described above.
However, it has not hitherto been possible to adequately and fully exploit said insulating materials, particularly for direct current transmission. It is commonly believed that one of the main reasons for this limitation is the development and accumulation of so-called space charges in the dielectric insulating material when said material is subjected to a direct current. It is thought that space charges alter the distribution of the electrical field and persist for long periods on account of the high resistivity of the polymers used. The accumulation of space charges leads to a local increase in the electrical field, which is consequently greater than that which would be expected considering the geometrical dimensions and the dielectric properties of the insulating material.
The accumulation of space charges is a slow process: however, the problem is accentuated when the direct current transported by the cable is reversed (in other words, if there is a reversal of polarity). As a result of this reversal, a capacitive field is superimposed on the whole electrical field and the value of the maximum gradient can be localized within the insulating material.
It is known that a prolonged degassing treatment, which may be carried out, for example, by subjecting the insulating material based on a cross-linked polymer to high temperatures and/or to a high vacuum for a long period, makes it possible to obtain an insulating material which is capable of limiting the accumulation of space charges when the cable is subjected to polarity reversal. In general, it is thought that, by virtue of the removal of the decomposition products of the cross-linking agent (for example dicumyl peroxide which forms acetophenone and cumyl alcohol on decomposition) from the insulating material, said degassing treatment reduces the formation of space charges. However, a prolonged degassing treatment obviously leads to an increase in the production times and costs.
In efforts to reduce the accumulation of space charges, it is known practice to modify cross-linked polyethylene (XLPE) by introducing small amounts of polar groups.
Patent application EP-A-0 463 402 discloses an ethylene (co)polymer containing polar groups chosen from ketone, nitrile and nitro groups in an amount of between 20 ppm and 8000 ppm, said polar groups having a dipole moment of greater than 0.8 debye. Said (co)polymer is said to be usable as an insulating material for high voltage cables with improved dielectric rigidity. Said polar groups may be introduced into the polyethylene by various processes such as, for example:                by copolymerization of a comonomer containing said polar groups with ethylene;        by blending an ethylene polymer or copolymer containing said polar groups with a conventional polyethylene;        by oxidation of a conventional polyethylene;        by grafting comonomers containing said polar groups onto a conventional polyethylene.        
Japanese patent application JP 10/283 851 discloses a cable for direct current transmission which has improved dielectric rigidity, in the presence of polarity reversals or following applications of electrical pulses, in which the insulating coating consists of a polymeric composition comprising a cross-linked polyolefin containing (i) a dicarboxylic acid anhydride and (ii) at least one monomer containing a polar group (chosen from at least one carbonyl, nitrile or nitro group). A particular peroxide, more specifically 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and a particular antioxidant, more specifically a thiocarboxylic acid ester, are required. Said groups (i) and (ii) can be introduced into the polyethylene by various processes such as, for example:                by copolymerizing a comonomer containing said groups with an olefin (for example ethylene);        by grafting said groups onto a polyolefin;        by blending polyolefins modified by grafting with the above groups (i) and/or (ii) with a polyolefin as such.        
The abovementioned composition is prepared by mixing the peroxide, the antioxidant and the modified polyolefin according to one of the above-mentioned processes and by heating to bring about the cross-linking.
Japanese patent application JP 06/215 645 discloses a cable for high voltage direct current transmission which shows a reduced accumulation of space charges. The insulating coating is prepared by hot cross-linking of a blend of a polyethylene, an organic peroxide having a half-life at 130° C. of greater than 5 hours and an acid chosen from itaconic acid and crotonic acid in an amount of less than 5 parts by weight per 100 parts by weight of polyethylene.
Japanese patent application JP 05/266 724 discloses a cable for high voltage direct current transmission with a reduced accumulation of space charges. The insulating coating is prepared by:                adding to the polyethylene a compound chosen from, for example, vinyl acetate, benzoic acid, naphthoic acid, acrylic acid; or        hot cross-linking of a blend of polyethylene, an organic peroxide having a half-life at 130° C. of greater than 5 hours and a compound chosen, for example, from vinyl acetate, benzoic acid, naphthoic acid, acrylic acid.        
Said compound is present in an amount of up to 10 parts by weight per 100 parts by weight of polyethylene.
As mentioned above, some of the known prior art solutions concerning insulating coatings, particularly insulating coatings for cables for high voltage direct current transmission with a reduced accumulation of space charges, envisage the use of cross-linked polyolefins, particularly cross-linked polyethylene, which have been modified beforehand by introducing polar monomers by means of copolymerization or pre-grafting. The Applicant believes that, when the polyolefins are modified by copolymerization, the introduction of the polar monomer presents a number of difficulties due to the fact that said polar monomer tends to form blocks and is therefore not evenly distributed along the polymer chain. When the polyolefins are modified by pre-grafting with said polar monomers using a radical initiator (for example an organic peroxide), since it is necessary to work at low concentrations of said radical initiator so as to avoid scorching of the polyolefin, said monomers have a tendency to homopolymerize. The presence of homopolymers may give rise to zones rich in polar domains which promote the accumulation of space charges.
Other solutions instead propose to carrying out hot cross-linking of a composition comprising polyethylene as such, an organic peroxide and a compound containing polar groups. However, the Applicant believes that, in this case also, a number of problems may be encountered since, by working as disclosed in the abovementioned prior art, besides the problems associated with the formation of homopolymers, the insulating coating thus obtained may contain unreacted compounds which are difficult to remove by the usual degassing techniques since they are, for example, relatively non-volatile. Also the presence of unreacted compounds may have an adverse effect on the properties of the insulating coating thus obtained by promoting the accumulation of space charges.
The Applicant has also found that the use of excessive amounts of compounds containing polar groups, instead of reducing the accumulation of space charges, has a tendency to increase said accumulation, thus reducing the electrical performance qualities of the cable thus obtained.